1. Field of the Invention
This invention relates to borehole logging tool cryostats for high-sensitivity photon detectors.
2. Description of the Prior Art
A technique of particular value in the exploration for oil or other hydrocarbons in sub-surface earth formations traversed by a borehole is that of gamma ray analysis. However, this technique frequently requires the use of high resolution semiconductor-crystal detectors, for example of high purity germanium (HP Ge) or lithium-drifted germanium (Ge (Li)), which operate reliably only at cryogenic temperatures, for example of the order of 90K (-183.degree. C.). The high ambient temperatures typically encountered in borehole logging up to and above 473K (+200.degree. C.), have thus been a serious obstacle to the use of semiconductor-crystal detectors in borehole exploration. Furthermore, space and other limitations place severe constraints on the techniques that may be applied in the design of cryostats for borehole logging tools, or sondes.
Much work has been done in the past on the maintenance of desired temperature conditions in equipment intended to operate in the borehole environment. This work has generally involved the use, alone or in combination, of conventional techniques of insulation, refrigeration, or cryogenic heat absorption.
Initially, the purpose of such work was merely to prevent equipment such as photomultiplier tubes from rising to temperatures at which damage could result, or to maintain temperature-sensitive devices at a uniform temperature. Thus, the target temperatures were at, or even somewhat above, ambient temperatures on the ground, and not the cryogenic temperatures required for high-resolution detectors. Subsequent work on the maintenance of true cryogenic temperatures in borehole equipment has involved the use either of a liquified-gas cryogen such as liquid nitrogen, or of a melting-solid cryogen such as propane or Freon-22 which is itself frozen using liquid nitrogen prior to a logging run. However, the use of such cryogens involves various problems.
A liquified-gas cryogen continuously evolves vapor. Additionally, there is always a significant risk of a borehole logging tool becoming stuck for many hours in a borehole, at an ambient temperature of as much as 423K (+150.degree. C.). Under these circumstances, even a melting-solid cryogen will ultimately change entirely to the gaseous state. In view of the high pressures encountered in deep boreholes, arrangements to vent vaporized cryogen may not operate effectively, thus creating a risk of explosion of the cryostat. Although it is possible to provide an expansion chamber to accommodate the vapor, a chamber of adequate volume, but sufficiently narrow to enter a borehole, would have to be several meters in length, and would be somewhat impracticable.
Accordingly, prior to the development of the present invention, conventional cryogenic techniques have been difficult or impossible to use in the context of a borehole logging tool, or have been unable to maintain sufficiently low temperatures for a period of adequate duration. Therefore, the art has sought a borehole logging tool cryostat capable of maintaining cryogenic temperatures without the problems presented by the prior art.